Which gas is carried in cylinders by mountaineers?

Oxygen 

The correct answer is option 1. Oxygen.

Mountaineers carry cylinders filled with oxygen gas.

At high altitudes, the air becomes thinner, meaning there is less oxygen available. This can lead to altitude sickness, which can be dangerous and even deadly. By carrying oxygen, mountaineers can supplement the reduced oxygen and breathe more easily, improving their performance and safety during their climbs.